72 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



Basals (BB) . The plates which collectively form a circlet just below the radials ; they 

 are usually five in number and alternate in position with the radials, but many 

 forms possess only three ; they may be entirely distinct, with the suture lines 

 easily visible between them, or they may be solidly anchylosed, forming a solid 

 ring or funnel (see figs. 2, p. 61, 3, p. 62, 14, p. 65, 115-118, p. 183, 122, p. 191, 

 130-134, p. 203, 144, p. 207, 145, p. 209, and 407-413, p. 317, and pp. 316-331). 

 In the recent comatulids the basals, at first forming an essential part of 

 the calyx wall, become in early life metamorphosed into the rosette and there- 

 fore disappear from external view, except in the family Atelecrinidas where 

 they are almost always to be seen forming a narrow ring between the centro- 

 dorsal and the radials (see figs. 123, p. 192, 124, 125, p. 193, 414, p. 319, and 430, 

 p. 321, and pi. 8, figs. 573-575, and pp. 318-320). 



Many recent comatulids have, just above the centrodorsal in the interradial 

 angles, more or less pronounced tubercules which are often so large as to appear 

 as true basals ; these are, however, Basal rays of secondary origin, and have no 

 connection with the larval basals (see figs. 415, p. 319, and 416-427, p. 321, and 

 pp. 326-330). 



In the recent crinoids the infrabasals, when present, form a circlet within 

 the basals and are entirely concealed by them; in the comatulids they fuse with 

 the uppermost columnal in early life to form the centrodorsal, or are entirely 

 absent (see figs. 565-572, pi. 7, and pp. 313-316). 



The basals are the equivalent of the genitals in the echinoids. 



Basal surface. Of the centrodorsal ; the dorsal pole. 



Base. (1) Of the calyx; the Radial pentagon; 



(2) Of the centrodorsal, the surface which is applied to the radials (see 

 figs. 229, 230, 232-234, p. 247, 235-242, p. 249, and p. 232). 



Bifascial articulation. Same as Synarthry. 



Bilateral symmetry. See Symmetry and Axis. 



Bivium. A term used to designate the posterior pair of arms, or rays, when these 

 differ from the three anterior in being short, ungrooved, and nontentaculiferous, 

 as in many of the Comasteridse (see figs. 45a-b, p. 79, and pp. 110, 111). 



Bourgueticrinoid stem. A stem or column of the t}'pe found in the species of the 

 genus Bourgueticrinus, This type of stem is characteristic of the young of 

 the comatulids and of the pentacrinites (see figs. 135-139, 141-143, p. 205, 

 518-524, 526, pi. 1, and 527, pi. 2, and pp. 208-210). 



Brachial ambulacra. The ambulacra on the ventral surface of the arms and of the 

 division series (in contrast to those of the disk and the pinnule ambulacra) (see 

 fig. 45a, p. 79, and pp. 110, 111). 



Brackial axillary. A term used by some authors for any of the axillaries except the 

 first, which is differentiated as the IBr, primibrachial, radial, or costal axillary. 



Brachial perisome. The perisome upon the ventral surface of the arms, beyond the 

 second brachial. 



Brachials (Br) . The calcareous segments or ossicles of which the arms are composed ; 

 many authors have used this term for all the ossicles beyond the radials, but 

 it is more properly used, as herein, for the ossicles beyond the last division 

 series only (see fig. 1, p. 60, fig. 2, p. 61, and fig. 6, p. 63). 



